The invention relates to a fermentation process for producing xanthan gum using bacteria of the genus Xanthomonas. 
Xanthan, or xanthan gum, is excreted by bacteria of the genus Xanthomonas, for example Xanthomonas campestris NRRL B-1003, NRRL B-1459, NRRL B-1043, or related organisms. The length of a xanthan molecule varies as a function of the: i) bacterium strain used, ii) composition of culture media, and iii) downstream processing; its average molecular weight is reported in the range of 2×106 to 5×107 Daltons (Da). The xanthan primary chain is made of β-d-glucose monomers; the side chains are linked to the primary chain at every second glucose and are composed of one glucuronic acid sited in-between two D-mannose.
Industrial production of xanthan started in the mid 1970s and today the hetero-polysaccharide is widely used as an emulsifier, stabilizer, thickener, and friction and water mobility reducer in industries as different as food and petroleum. Numerous xanthan based products have been developed, for example: i) xanthan-cryogels, which could replace gelatin in the food industry thereby eliminating potential BSE related problems while meeting the needs of vegetarians and populations not consuming cattle or pigs, and ii) xanthan-hydrogels, which could be used in pH and similar sensors. With an estimated worldwide production of over 40,000 tons per year and a price of about $7-8 per kilogram (as reported in 2002), xanthan is the most important, industrially produced, microbial hetero-polysaccharide.
In industry, xanthan is usually produced by aerobic submerged fermentation conducted in a batch-mode for up to five days. To minimize oxygen transfer and viscosity problems in the submerged fermentation, the broth is either made of a very large amount of water and a relatively low concentration of nutrients or a very large amount of water and a sequential addition of nutrients, which results in a lower concentration of nutrients. The fermentation process requires a carbon source, which is commonly glucose, sucrose, or starch in concentrations of 1-5%. The concentration and the type of carbon source and the carbon-to-nitrogen ratio of the substrate are important because these factors affect the xanthan yield. After fermentation, xanthan is precipitated from the pasteurized fermentation broth with ethanol, methanol, isopropyl or similar alcohols. Not less than two, but often more than three volumes of alcohol per volume of broth may be required for precipitation. Therefore, large amounts of solvents are needed to extract xanthan from the fermentation broth because of the high water content of the broth. Following extraction, the raw xanthan is dried, milled, and packed for further use or purification.
Alternative carbon sources have been tested on xanthan fermentation, for example: apple pomace, spent malt grains, citrus waste, olive-mill wastewater, sugar beet pulp, and whey. Solubilized residues of cassava, cassava bagasse, potatoes, husks and pulp of coffee beans have also been tested on xanthan fermentation. With the exception of the citrus waste, apple pomace, and spent malt grains works, all other works on xanthan fermentation using residues from the agro- or food industry were conducted following the release of simple sugars from the residues by means of an enzymatic, acid, or base treatment. Additionally, a number of improvements concerning aerobic submerged fermentation of xanthan have been developed to increase xanthan yield.
Xanthan fermentation is characterized by a significant increase in the viscosity of the fermentation medium, which limits the transfer of oxygen and other chemicals to the Xanthomonas campestris bacteria while decreasing both the xanthan yield and quality. It is highly desirable to maintain the appropriate supply of oxygen and other chemicals to the bacteria while minimizing the energy requirements pertinent to oxygen supply and mixing caused by utilization of power intensive aerating and mixing devices. It is also highly desirable to increase the xanthan production while decreasing, preferably eliminating in full, the large volumes of water involved in aerobic submerged xanthan fermentation. Such fermentation would have a higher yield of xanthan, require much less solvents for extracting the xanthan from the broth, and, consequently, yield substantial economical, environmental, and technological benefits for xanthan producing plants and is the subject of this invention.